Since the demise of Air Turquoise, which operated from Luton to Reims Champagne Aerodrome, CDG is now the nearest airport (served from the UK) to Soissons.
Travel from CDG to Gare du Nord is really easy. Simply hop on any RER train and it will take you straight there. (CDG is at the end of RER Line B, which is the only RER line serving the airport. So it's impossible to get on the wrong line, or to take a train in the wrong direction. If it's an RER train leaving CDG, it will definitely go to Gare du Nord).
Train times to Soisson can be found here:
http://www.raileurope.co.uk/
As Pigleton suggests, Eurostar might be easiest (and, quite possibly, the cheapest). The Eurostar website offers Leeds as a starting point but doesn't list Soissons among the destinations where through ticketing is available online. It could be worth phoning them to check. When booking well in advance it's usually fairly easy to find a fare of £59 return from St Pancras International to Gare du Nord. Adding on domestic travel often only adds about £40 on for return travel from any UK station to any station in France. (Rail travel from Leeds would simply involve walking across the road from Kings Cross station to St Pancras International in London, and changing platforms at Gare du Nord in Paris).
Accommodation in Soissons can be found here:
http://uk.tourisme-soissons.fr/hebergement
Chris